U.S. Pat. Nos. 4,009,511, 4,056,652 and 5,082,611 disclose processes for making heavy denier, high tenacity polyamide monofilaments which are well suited for use in reinforced rubber goods such as tires. These processes include the steps of spinning, water-quenching, and drawing a heavy denier, polyamide monofilament in at least first and second draw stages. In the first draw stage, the quenched monofilament is advanced through a steamer containing a high temperature steam atmosphere and is advanced in the second draw stage through a zone heated with a radiant heater. The monofilament is drawn to a total draw ratio of at least about 5.5.times.. The monofilaments produced by these processes have a surface with an orientation less than the orientation of the core which, besides imparting improved physical properties to the monofilaments, provides good adhesion to rubber.
For achieving very high tenacities, i.e., greater than about 9 gpd, using processes of this type, it has been discovered that it is desirable for the extent of crystallization in the quenched monofilament to be low so that the monofilament can be drawn to higher draw ratios. This low crystallinity can be accomplished by very rapid cooling of the filament in cold quench water with an extended residence time so that the monofilament core temperature is cooled to below about 55.degree. C. However, when the temperature of the core of the quenched monofilament is below about 55.degree. C., problems can arise in the first draw stage. Rather than the desired single "neck" draw at the draw point, a series of separated necks may form that "run together" as the draw is completed. This type of draw results in low and variable tensile properties and poor spinning continuity and becomes more prevalent in higher denier monofilament as the thickness becomes greater since it is usually necessary with thicker filaments to get the surface and average temperature of the monofilament well below about 55.degree. C. so that the core temperature is below about 55.degree. C.
The process disclosed in U.S. Pat. No. 5,082,611 provides a method for controlling the location of the draw point and can provide a standard deviation of tenacity of less than about 0.25 in high tenacity monofilament. This process utilizes controlling the temperature of the quenched monofilament in advance of the steamer by adjusting the length of time in the quench bath or by regulating the quench bath temperature so that the draw point is maintained after the feed rolls and before the high temperature steam atmosphere. A preferred location for the draw point is in the steam expansion zone of the high pressure steamer. However, it is often difficult to provide the proper quench conditions which achieve both the low crystallinity needed for very high tenacities and the desired control over the location of the draw point to provide uniformity. Small perturbations in the process with time can result in movement of the draw point from outside to inside the steamer resulting in less than desired tensile strength and/or tensile strength uniformity.
Control over the draw point becomes especially difficult for higher denier monofilaments because the increased thickness requires more extreme quenching and lower surface and average monofilament temperatures to achieve low crystallinity. Thus, the draw point of the cooler monofilaments will tend to occur farther downstream in the process and it is sometimes difficult to avoid having part of the draw point within the high pressure steam zone. When this occurs, the steam penetrates too far into the monofilament surface causing deorientation and thus lower overall tensile strength of the yarn. In monofilaments where the minimum thickness of the monofilament when quenched is greater than about 0.8 mm, it has sometimes been found that quench conditions cannot be adjusted to provide both the desired low crystallinity and control of the draw point location at desirable process speeds.